Heat resistant alloy for carburization resistance

ABSTRACT

The catastrophic carburization of metal furnace tubes used in cracking and pyrolyzing hydrocarbons is minimized by forming a protective scale of manganese and chromium oxide on the surface of the metal. A nickel content of about 36 to 38 percent and a manganese content of 1.25 to 2.0 percent in a nickel-chromiumiron alloy promotes the formation of the oxide scale upon treatment of the alloy with steam.

United States Patent n 1 Bagnoli et al.

[ HEAT RESISTANT ALLOY FOR CARBL'RIZATION RESISTANCE [75] Imentors: Donald L. Bagnoli, Oak Ridge:

Anthony R. Ciuffreda, COlOfiltL both of NJ.

[73] Assignee: Exxon Research and Engineering Company Linden, NJ.

[22] Filed: Aug. 28. i974 [2|] App]. No: SOLZZO Related US. Application Data [62] Dnision olSer No 3X7,)4(1 Aug l3. I973 Put. No

.RSfiiM-t.

[52] L15. Cl. 208/47; 208/48 R {5|} lnt. Cl. C100 9/l6; ClOG 9/22 [58] Field of Search w. 208/47, 48

[561 References Cited UNITED STATES PATENTS lJtllU-W 3/193) Norwood r r r r v i i i i v r ZEN/6X3 NOV. 11, 1975 2.543.710 2/195! Schmidt et ul Y r H Hit/(x15 167M150 3/I954 Shortsleew et ul.. .7 19ml 35 290m 9/1959 ShilplLighi 21mm 3.260.594 7/l966 Ornitz et ill i i v i i. 757138 A Prl'mm' EmminerDclbert E. Gantz Assisnmr Examiner-G. E. Schmitkons Attorney, i-lgenl 0r FirmJoseph J. Dvorak [57] ABSTRACT 3 Claims. N0 Drawings HEAT RESISTANT ALLOY FOR CARBURIZATION RESISTANCE RELATED APPLICATION This is a division, of application Ser. No. 387,946, filed Aug. 13, 1973, now US. Pat. No. 3,865,634.

BACKGROUND OF THE INVENTION This invention relates to the pyrolysis of hydrocarbons. More particularly, it relates to a technique for minimizing carburization of furnace tubes exposed to hydrocarbons under pyrolysis and reforming conditions.

Processes for the controlled pyrolytic decomposition of hydrocarbons are well known. Typically, the hydrocarbon is decomposed at high temperatures, for example in the range of 1,700 to 2,000F. under varying pressure conditions ranging from atmospheric up to about 1,200 psi. Such pyrolytic techniques are frequently used, for example, in producing ethylene. The hydrocarbons are cracked at elevated temperatures in the presence of predetermined amounts of steam. Pyrolytic techniques are also used for increasing the octane rating of gasoline.

It has been well recognized that the above-mentioned cracking and reforming processes when conducted in the presence of certain metals and metal alloys often lead to excessive deposition of carbon. These carbon deposits not only tend to reduce the size of the furnace tubes with concomitant problems of plugging, but most importantly they also affect the strength of the metal furnace tubes resulting in strucutral failure of the metal tubes. In such instances such failure is frequently referred to as catastrophic carburization of the metal.

It is know for example that iron surfaces will have a catalytic effect upon the pyrolysis of hydrocarbons, promoting carbon formation. The carbon formed is absorbed or diffused into the metal resulting in the structural failure of the metal. Consequently, numerous techniques have been proposed for avoiding such catastrophic carburization of metal tubes by alloying the iron and/or by forming protective coatings of an oxide on the surface of the metal to be contacted with the hydrocarbon under the reforming or pyrolysis conditions. None of these techniques have provided a complete panacea and there is more than definite commercial interest in prevent carburization of fumace tubes exposed to hydrocarbons under pyrolysis and reforming conditions.

SUMMARY OF THE INVENTION According to the present invention, there is provided a method of heating hydrocarbon carbons in contact with metal surfaces under conditions which normally would cause difficulties clue to carburization of the metal surfaces which comprises contacting the hydrocarbons of the metal surface containing adherent oxide layer of manganese and chromium, whereby heat is passed from the metal surface to the hydrocarbon without significant carburization of the metal.

Thus, in one aspect the present invention contemplates a method of rendering metal surfaces exposed to hydrocarbons under pyrolysis and reforming conditions resistant to carburization by providing on such metal surfaces in contact with the hydrocarbons an adherent layer of manganese and chromium oxide.

2 In yet another aspect of the present invention an adherent layer of manganese and chromium oxide is formed on a metal alloy surface by oxidizing a nickelchromium-iron alloy that has at least 36 percent nickel and from 1.25 percent to 2.0 percent manganese.

DETAILED DESCRIPTION OF THE INVENTION The present invention can be carried out for example by passing a hydrocarbon through a heating means having one or more tubes or conduits which are heated directly or indirectly to transfer heat to the hydrocarbon. The metal surfaces to be used in accordance with this invention should have an adherent coating of manganese and chromium oxide.

In a particularly preferred embodiment of the present invention the metal tubes are formed from an alloy consisting essentially of chromium, nickel and iron and having at least 1.25 percent manganese and at least 36 percent nickel. For example, the alloy may have from 36 to 38 percent nickel, from 23 to 27 percent chromium, and from 1.25 percent to 2 percent manganese. Indeed such an alloy when contacted with steam at elevated temperatures for a time sufficient to oxidize some of the manganese present in the alloy results in the formation of an adherent protective coating of manganese and chromium oxide which is resistant to carburization. For example, the metal surfaces are pretreated with steam at temperatures in the range of 500F. to about 2,000F., and preferably at about l,500F. for from about 24 hours to about 96 hours, such as about 72 hours.

Equally important in providing an adequate protective coating of manganese and chromium oxide on the surface of the metal is the grain structure of the contact surface. It has been discovered that with furnace tubes, for example, the interior contact surface should be made up of equiaxed grain structure. Basically the grain structure is achieved by casting the alloy into a tube and controlling the thermal gradient during solidi fication. Any other technique known in the art for controlling grain structure can be employed.

The invention will be better understood by reference to the following examples and demonstrations.

EXAMPLE I TABLE I Alloy l Alloy 2 Carbon, 0.4-0.5 0.43 Manganese, *2 L09 L30 Silicon. I.0l LIO Chromium, 28.0 23.5 Nickel, l9.0 37.0 Molybdenum, 0 L89 Iron, Balance Balance At the end of 30,000 hours on stream the furnace tubes were physically inspected. Those tubes of alloy 1 were, at least in some instances, carburized as much as 3 80 percent and had as little as 2/3 of an effective wall thickness remaining. Those of alloy 2 were effectively protected against carburization. Magnetic readings of the tubes of both alloy 1 and alloy 2 showed that the protected regions were predominantly oxides of manganese and chromium whereas the unprotected areas were predominantly oxides of iron. v

in a more detailed analysis performed with a microprobe analyzer, it was surprisingly discovered that a continuous layer of manganese oxide was lying outside the chromium oxide layer and close to the surface and the highly protected layers. Indeed, in alloy 2 the thickness of the manganese oxide scale was about times greater than the amount of manganese oxide scale in alloy 1.

Apparently, a nickel content, in a nickel-chromiumiron alloy, of greater than 36 percent and a manganese content of greater than L percent is necessary to provide a good protective scale of manganese and chromium oxide in the interior surface of the furnace tube.

Although the present invention has been described in conjunction with preferred embodiments, it is to be un 4 derstood that modifications or variations may be resorted to without departing from the spirit and scope of the invention. Those skilled in the art will readily understand that such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

What is claimed is:

l. A method for heating hydrocarbons by contact with heated metal surfaces containing nickel and iron under conditions which are likely to result in carburization of the metal, which comprises contacting said hydrocarbons with a heated metal surface that has an adherent layer of manganese and chromium oxide on the metal surface which is in contact with the hydrocarbon, whereby the metal is protected against carburization and heat is passed from the metal to the hydrocarbon.

2. The method of claim 1 wherein said hydrocarbon is contacted with said metal surface at an initial temperature in the range of about l,700F. to 2,000F.

3. The method of claim 1 wherein the metallic surface comprises the inner walls of heat exchanger tubes suitable for heating said hydrocarbon. 

1. A METHOD FOR HEATING HYDROCARBONS BY CONTACT WITH HEATED METAL SURFACES CONTAINING NICKEL AND IRON UNDER CONDITIONS WHICH ARE LIKELY TO RESULT IN CARBURIZATION OF THE METAL, WHICH COMPRISES CONTACTING SAID HYDROCARBONS WITH A HEATED METAL SURFACE THAT HAS AN ADHERENT LAYER OF MANGANESE AND CHROMIUM OXIDE ON THE METAL SURFACE WHICH IS IN CONTACT WITH THE HYDROCARBON, WHEREBY THE METAL IS PROTECTED AGAINST CARBURIZATION AND HEAT IS PASSED FROM THE METAL TO THE HYDROCARBON.
 2. The method of claim 1 wherein said hydrocarbon is contacted with said metal surface at an initial temperature in the range of about 1,700*F. to 2,000*F.
 3. The method of claim 1 wherein the metallic surface comprises the inner walls of heat exchanger tubes suitable for heating said hydrocarbon. 